The improvement of many plants, such as turfgrass, through conventional breeding usually relies on the identification of a single improved trait within a cultivar and is restricted to germplasm that is capable of sexual crosses to yield fertile offspring. An improved trait within a given cultivar once identified, is followed by extensive backcrossing, selection and evaluation to produce a commercially viable product. This process can require up to fifteen years, and is restricted to traits confined to the gene pool of the plant.
In contrast, many important crop plants are genetically transformed with genes from other species, even across kingdom barriers. The introduction of cloned genes into plant cells and recovery of stable fertile transgenic plants can be used to make modifications in a plant, and has created the potential for genetic engineering of plants for crop improvement. Genetic modifications by plant transformation allow stable alterations in biochemical processes that direct traits such as increased yield, disease and pest resistance, herbicide tolerance, nutritional quality, drought and stress tolerance, as well horticultural qualities such as pigmentation and growth, and other agronomic characteristics for crop improvement. In these methods, foreign DNA is introduced into the eukaryotic plant cell, followed by isolation of cells containing the foreign DNA integrated into the cell's DNA, to produce stably transformed plant cells.
One drawback that arises regarding transgenic improvement of perennials, such as turfgrass, is the possibility for transgene escape to wild and non-transformed species. For example, creeping bentgrass is an out-crossing and wind pollinated, stoloniferous, perennial species. These traits can increase the risk of outcrossing, persistence, and introgression of alien genes into an adjacent population. However, the bulk of most of the risk assessment work conducted on transgenic plants has been on annual and/or self-pollinating crops. As a result, there is a lack of information on the potential risks from the commercialization and large-scale seed production of these types of transgenic crops. In a three-year field study on gene-flow of transgenic bentgrass, it was observed that pollen from the transgenic nursery traveled at least 411.5 feet (Wipff and Friker, Diversity 16:36-9, 2000). Therefore, there is a need to develop methods which decrease, or even prevent transgene escape in perennial plants.
Traditionally, male sterility has been used in crop plants for the production of hybrid varieties with higher yield, increased resistance to disease, and enhanced performance in different environments compared with the parental lines. In both corn and rice, cytoplasmic male-sterile (CMS) plants were successfully used on a large scale in hybrid seed production. However, the use of a unique type of cytoplasm has drawbacks because of the increased vulnerability of the plant to insects and pathogens (Levings, Science 250:942-7, 1990). Therefore, alternative methods for producing hybrid plants are needed. In corn, detasseling (removal of the male part of line A) and then pollination by line B for hybrid production is an alternative, but detasseling is costly. In addition, neither detasseling nor CMS systems are available for many economically important crops, such as wheat, soybean, canola, or barley, which are still bred and grown as inbred varieties.
An alternative to CMS and detasseling is the development of male sterility by the selective ablation of tapetal cells, which are important for pollen development (Mariani et al., Nature 357:384-7, 1990; Moffatt and Somerville, Plant Physiol. 86:1150-4, 1988; Tsuchiya et al., Plant Cell Physiol. 36:487-94, 1995; Xu et al., Proc. Nat. Acad. Sci. USA 92:2106-10, 1995). Selective ablation of tapetal cells by cell-specific expression of cytotoxic molecules (Mariani et al., Nature 357:384-7, 1990) or an antisense gene (Xu et al., Proc. Nat. Acad. Sci. USA 92:2106-10, 1995) blocks pollen development, resulting in male sterility in annuals. However, methods to restore the fertility for the antisense-caused male sterility have not been developed. Luo and Hodges (Plant J. 23:423-430, 2000), using a site-specific recombinase eliminated, in transgenic male-sterile plants, the male-sterility causing elements integrated into the host genome by crossing with pollen from a plant expressing a recombinase, yielding hybrid seeds.